Origin and tectonic implications of Late Jurassic high-Mg diorites along the Bangong-Nujiang suture zone, Tibet

ABSTRACT

The Shiquanhe-Num Co ophiolitic mélange belt (SNOB) is one of two such belts along the Bangong-Nujiang suture zone (BNSZ) separating the Lhasa and Qiantang terranes of central Tibet. This study reports new geochronological, petrological and geochemical data for high-Mg dioritic dikes intruding the Shiquanhe ophiolitic peridotite. Based on LA-ICP-MS zircon U–Pb dating, dioritic dikes yield a Late Jurassic age of 162.8 ± 1.3 Ma. These are high-Mg diorites (53.0%–56.8% SiO₂, 7.07%–8.82% MgO) with moderate TiO₂ (0.95%–1.01%), high K₂O+Na₂O (6.02%–8.01%), and high contents of Cr (150–182 ppm), Ni (91–145 ppm) and Th (2.8–5.9 ppm). They are very similar to sanukites of the Setouchi volcanic rock belt (SW Japan). Shiquanhe diorites have high Sr/Y ratios (up to 41.3), distinct positive Pb anomalies, negative Nb anomalies and continental crust-like trace element patterns without negative Eu anomalies (average Eu/Eu* = 0.96). These geochemical characteristics indicate that Shiquanhe diorities were derived by partial melting of mantle peridotite as a result of interaction with hydrous melts of subducted sediments. Similar high-Mg magmas are common in the early evolution of a new subduction zone. Integrated with field observations and results from other studies of Late Jurassic high-Mg andesites and diorites along the SNOB, we argue that the Shiquanhe high-Mg diorites formed at an early stage of northward subduction of the Bangong-Nujiang Tethys ocean (BNO) beneath the Qiangtang terrane. Therefore, these results provide critical constraints on the Jurassic tectonic evolution of the Shiquanhe-Num Co ophiolitic mélange belt and are significant for understanding the evolution of the BNO.

Graphical Absrtact

KEYWORDS:

• Tibet
• igneous geochemistry
• BNSZ
• high-Mg diorite
• subduction

Highlights

• The Shiquanhe ophiolitic belt is the key to clarify the evolution of BNSZ

• High-magnesium andesites (HMAs) closely related to the subduction initiation

• 160-165 Ma HMAs were common in the forearc of the Southern Qiangtang

• Northward subduction initiation occurred in the Shiquanhe area in ~163 Ma

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed here.

Additional information

Funding

This study was co-supported by funds from National Natural Science Foundation of China [No. 41102065 and No. 41872110], China Geological Survey [DD20160026], Sichuan Science and Technology Program [No. 2018JY0465], and Chengdu University of Technology [10800-18Z0102]. This is UTD Geosciences contribution #1366.